1. Field of the Invention
The present invention relates to a toner composition for use in developing an electrostatic latent image, a method for its preparation, and a method for charging the toner composition. More particularly, the invention relates to toner components which, when used within a specific type of toner development system, provide a unique blend for application in a field charging method to exercise proper charging levels and stability.
2. Description of the Related Art
Toners including a polymeric resin and a coloring agent are known in the art of developing electrostatic images. Often, the toner is post-blended with colloidal silica or fine silica powder to improve the flowability of the toner. It is commonplace in the art to charge a toner before it is used to develop an electrostatic image. The method that has been traditionally used to charge the toner is triboelectrification. In the toning systems where the toner is charged triboelectrically, this is accomplished by rubbing of the toner particles against the carrier beads, against dissimilar surfaces inside the toner reservoir, or against a charging blade. This charge comes to a preset level determined by the time of agitation, the materials involved, and the level of agitation. When the toner attains that charge, there is no variability, i.e., the imaging system and toning system are optimized to a level of charge, one image potential on the imaging member of the system, and one speed. Any ratio of charge in the system (charge-to-area on the toner, charge-to-mass on the toner, and mass-to-area on the delivery rollers and latent image) is fixed. In other words, the development system and toner of the prior art are non-transportable to other imaging systems without significant modification and system optimization.
The present inventors have found that with the toner and charging method of the invention, the process is capable of a variable charge. With this capability, variable charge-to-area ratios on the toner are possible along with variable charge-to-mass ratios. From this, the mass-to-area coverage on the toner delivery rollers can be controlled. The result is control over the toner which allows imaging from 30 to 300 feet/minute with no change in image density or quality. This will be described in further detail below.
The toner and charging method of the present invention also have an advantage over the prior art in that once the toner formulation is established, the process can be expected to operate in a similar fashion regardless of the toner coloration. In contrast, a tribocharged toner of the prior art depends heavily on the chemical composition of the toner for the amount of charge it can develop, the pigmentation of the toner being integral to this composition. In short, with tribocharged toners, the charge level is very dependent on the coloration the toner. To have a color toner operate similar to another color within the same system requires considerable adjustment to other additives in the base toner. The present invention eliminates this problem by providing a charging process which is insensitive to toner formulation.
The prior art heretofore has not recognized the importance of the bulk (volume) resistivity of a toner composition to maintaining the charge distribution in a toner system, which is one aspect of the present invention discussed below. Further, the prior art does not teach charging a toner composition by suspending toner composition particles in air followed by subjecting the suspended toner particles to field charging.
Perhaps most importantly, there is no teaching or suggestion in the prior art of a toner which can be used in various imaging systems without modification and which can carry different pigmentation types without affecting its charge. Yet, such a toner and charging method would result in an even charge distribution, offering the advantage of a lowered electrostatic adhesion force, thus making image development more efficient by allowing for higher speed operation.